1. Technical Field
The invention relates to a voltage generator, and a more particularly to an asymmetric voltage generator.
2. Related Art
FIG. 1 is a circuit diagram of a conventional voltage tuner 100. The voltage tuner 100 includes an operational amplifier 110, a transistor MP, and the resistors Rf1 and Rf2. The operational amplifier 110 has a positive input terminal receiving an input voltage Vref, and a negative input terminal receiving a feedback voltage Vf transmitted back between the resistors Rf1 and Rf2. The transistor MP has a gate coupled to an output terminal of the operational amplifier 110, a source receiving a reference voltage Vin, and a drain connected to a terminal of the resistor Rf2 to generate an output voltage Vout. Another terminal of the resistor Rf2 generates the feedback voltage Vf, and the resistor Rf1 is connected in series between the terminal of resistor Rf2 generating the feedback voltage Vf and a ground voltage GND serving as another reference voltage.
The voltage tuner 100 is referred as a low drop-out (LDO) voltage tuner. Under a condition in which the feedback voltage Vf is equal to the input voltage Vref, a current Ip is equal to Vf/Rf1, and the output voltage Vout is equal to a product of the current Ip and a sum of the resistors Rf1 and Rf2. Therefore, in the voltage tuner 100, when the output voltage Vout is being adjusted, only the resistance of the resistor Rf2 needs to be altered.
It should be noted that, the voltage value of the output voltage Vout and the resistances of the resistors Rf1 and Rf2 are correlated. In order to ensure that the voltage value of the output voltage Vout is accurate, a layout of resistors Rf1 and Rf2 with stable resistances are required for the voltage tuner 100. Therefore, resistors Rf1 and Rf2 with greater widths are needed. On the other hand, in order to reduce the electric energy consumed by the resistors Rf1 and Rf2, these resistors are typically designed to have large resistances. Accordingly, the resistors Rf1 and Rf2 also require greater lengths. In other words, the circuit area occupied by the resistors Rf1 and Rf2 in the conventional voltage tuner 100 is very large which increases the circuit cost.